This invention relates to digitizers such as, or similar to, the digitizer disclosed in the aforementioned TRAVELLING WAVE DIGITIZER. That digitizer performs coarse and fine measurements by propagating an electrical bit pattern across a grid of conductors embedded in a platen. The coarse measurement involves propagating a long bit pattern at high speed; fine measurements require propagating a short bit pattern at low speed. Before a fine measurement can begin the remnants of the previous coarse or any other bit pattern must be entirely replaced by the fine bit pattern's propagation across the entire platen. This can take a significant amount of time, since the fine bit pattern is propagated at a slow speed. Furthermore, the longer the platen, the longer is the time required.
It is desirable to perform coarse/fine or fine/coarse measurement pairs as near together in time as possible, to reduce the effects of cursor motion and preserve the validity of the measurements. That is, if too wide a separation in time occurs between a coarse and fine measurement, the fine measurement might not reflect the fine location within the proper coarse interval, due to movement of the cursor between the measurements.
This difficulty can be avoided by ensuring that there is a minimum of time between coarse/fine or fine/coarse measurements. The present invention achieves this by rapidly shifting the fine bit pattern into place, and then shifting it at its proper slow speed. This minimizes the time required to begin a fine measurement.